Electronic devices such as an OLED display, which are thin, excellent in moving picture display performance, and low in power consumption, are used for applications such as a display for a portable phone.
A glass sheet is widely used as a substrate for an OLED display. A glass sheet used for this application needs to have the following main characteristics.
(1) To be substantially free of alkali metal oxides in order to prevent alkali ions from diffusing in a semiconductor film formed in a heat treatment process.
(2) To be excellent in productivity and be excellent particularly in denitrification resistance and meltability in order to produce a glass sheet at low cost.
(3) To have a high strain point in order to reduce heat shrinkage of a glass sheet in a production process for a p-Si.TFT.
The characteristic (3) is described in detail. The production process for a p-Si.TFT involves a heat treatment process at 400 to 600° C., and the heat treatment process causes a minute dimensional change, called heat shrinkage, in the glass sheet. When the heat shrinkage is significant, a pixel pitch shift occurs in the TFT, which causes a display defect. Along with emergence of a higher-definition display, even a dimensional shrinkage of about several ppm may cause a display defect. Accordingly, there is a demand for a low-heat-shrinkage glass sheet. Note that as a temperature of heat treatment to be applied to the glass sheet is higher, the dimensional shrinkage becomes larger.
As a method of reducing heat shrinkage of a glass sheet, there is known a method involving forming a glass sheet and applying annealing treatment to the glass sheet at around its annealing point. However, the annealing treatment requires a long time period, and hence production cost of the glass sheet soars.
As another method, there is known a method involving increasing a strain point of a glass sheet. The strain point is a characteristic which serves as an index of heat resistance. As the strain point is higher, the heat shrinkage is more unlikely to occur in the production process for a p-Si.TFT. For example, Patent Literature 1 discloses a glass sheet having a high strain point.